Myoblasts, myosins, MyoDs, and the diversification of muscle fibers.

Distinct types of muscle fibers form and become innervated by appropriate motor neurons during development. Though the activity pattern of the innervating motor neuron affects fiber type in the adult, it is now clear that innervation is not required for the initial formation of fast and slow muscle fibers during embryonic and fetal development. In addition, multiple types of intrinsically different myoblasts are found at different stages of development and motor neurons may preferentially innervate specific types of muscle fibers at relatively early stages of myogenesis. Thus, at least some of the information required for the formation of specific motor units must be carried by muscle cells. Cellular and molecular analyses of the multiple types of myoblasts, myosin heavy chain isoforms, and myogenesis regulating proteins of the MyoD family are leading to a new understanding of the events that choreograph the formation of fast and slow motor units.